When compared to an electrochemical device using conventional liquid electrolyte, electrochemical device using solid electrolyte has such advantages as having no solution leakage problem, and is processable into thin membranes and small sizes, thus it can be easily utilized in portable electronic devices and automobiles, etc.
Especially, a solid polymeric electrolyte thin membrane has been extensively studied and developed because it provides a chemical battery with high charging-discharging efficiency. Further chemical batteries made of such material may have various types and are lightweight.
For the production of a solid-type chemical battery of the above merits, currently polymeric compounds have been developed so as to be used as electrolytes, since polymers can be processed into thin membranes, can be used as an electrolyte due to their salt-dissolving property and ion permeability. When processed into the solid electrolyte, other advantages such as low battery resistance and high current flow even at low current distribution are also provided.
As described above, even though the polymeric ion-conducting thin membrane applicable for solid chemical battery has a good ion conductivity and mechanical property, it is difficult to improve both properties only by changing the physical properties such as the molecular weight of the polymer matrix and its glass transition temperature. Hence, it is required to develop a novel polymeric electrolyte having both improved physical property and ion conductivity.
In the U.S. Pat. No. 4,654,279, Baueretal. disclosed a battery using double network of a conducting liquid polymer, which has both a mechanical substrate consisting of a continuous network of a crosslinked polymer for improving the mechanical property of the solid electrolyte and two kinds of continuous phases consisting of an ion conducting phase providing the channel of ion transport through the matrix.
Le Mehaute et al. disclosed a solid electrolyte for an electrochemical material containing one or more polymers forming a complex and one or more ionizable alkali salt chelated with the above polymer. A process to produce the above solid electrolyte was disclosed which is characterized in that the above polymer forming complex was mixed to an amorphous state in a crosslinking process.
Xia et al. disclosed with regard to heat and ion conductability of a polymer ion electrolyte prepared by polymerization of oligoethyleneoxy methyl methacrylate (Solid state Ionics, 1984, 14, 221.about.224). However, as it is difficult to prepare a complete non-crystalline (amorphous) thin membrane from the polymer electrolyte of the above invention, the ion conductivity of the polymer membrane prepared varies greatly depending on its temperature. The ion conductivity at room temperature also changes with time; hence one has problems in applying it as material to practical use. The main chain of the polymer consists of only ethyleneoxy methyl methacrylate unit and thus the hardness of the membrane of the polymer polymerized is so large that it has a poor ion conductivity. Since the electrolyte membrane is brittle due to its poor mechanical property, one has difficulties in using the membrane for batteries or solid electrochemical materials. This is because the polymer electrolyte prepared by the above invention has low ion conductivity (less than 1.times.10.sup.-5 S/cm) and low adhesive strength for an electrode, when applying the electrolyte to a solid electrochemical device, one is faced with problems such as cracking of the electrolyte membrane and short life of the electrolyte membrane, etc., which are factors that would eventually shorten the life of the electrochemical device. Thus, there has been a need to develop a styrene-maleic acid polymer electrolyte having an excellent mechanical property and a rubber elasticity.
For example, Mellander et al. suggested a process to produce a solid electrolyte using a styrene-maleic acid polyethyleneoxy ester copolymer produced from compositions comprising styrene-maleic anhydride copolymer, polyethyleneglycol methyl ether and KOH as major components (Electrochimica Acta, 1995, vol. 40, 2413-2416). The polymer produced by the above invention has a solution processability, an excellent elasticity, but a very low ion conductivity (ion conductivity below 10.sup.-6 S/cm at room temperature). This is because the copolymer is produced as a type of union conductor using KOH, one is faced with a limitation when using this conducting thin membrane as a practical solid electrochemical material such as a lithium ion battery, etc. in practice.
Florianczyk et al. suggested a process to produce an electrolyte from compositions comprising styrene-maleic anhydride copolymer, polyethyleneglycol methyl ether and NaI as major components (Synthetic metals, 1990, 35, 249.). This electrolyte has also a solution processability and an excellent elasticity, but a very low ion conductivity (ion conductivity below 10.sup.-10 S/cm at room temperature) because it is produced as a type of union conductor using NaI. Thus, when using this conducting thin membrane as a practical solid electrochemical material such as a lithium ion battery, etc. in practice, one is faced with limitations.
Rietman and his collaborators suggested a process to produce an electrolyte from compositions comprising styrene-maleic anhydride copolymer, polyethyleneglycol methyl ether and lithium salt as major components (J. Polym. Sci., Part C: Polymer Lett. 1990. 28. 187). However, this electrolyte has also a very low ion conductivity (ion conductivity below 10.sup.-7 S/cm at room temperature) because it is produced as a type of union conductor. In addition, this electrolyte has a poor mechanical property.